Described herein is a ballistic resistant article, such as a helmet, comprising a double curved shell in turn comprising a stack of layers of an oriented anti-ballistic material, the layers comprising one or more plies each and having a plurality of cuts, the ends of which define a central polygon and lobes extending from the polygon, and wherein the stack comprises rotationally staggered layers, typically rotated about an axis extending through the centre of the polygon. The embodiments further relate to a semi-finished product for and method of making a shell for a ballistic resistant article.
Conventionally, ballistic resistant double curved articles, such as helmets, are manufactured using pattern moulding technology or draw/thermo forming technology. Both processes result in a shell of stacked layers that consist of anti-ballistic fibres embedded in a polymer matrix (˜15-25% w/w). Subsequently, the stack is consolidated by compression moulding and the polymeric matrix, for example a curing thermoset, e.g. phenolic resin, or a thermoplast, fuses into a unified entity. Due to matrix fusion, high matrix content and small fibre and ply dimensions, irregularities such as folding, overlap and gaps, the latter introduced by pattern cuts to facilitate adequate drapability, level off. Draw forming, described in US 2011/0159233, reduces the formation of irregularities when compared to pattern moulding, but is only feasible with reinforcing elements that can be drawn substantially at temperatures well below the melting temperature. Both technologies are successfully applied using ultra high molecular weight polyethylene (UHMWPE) fibres.
Recent advances in the development of high strength and high modulus tapes, using for example UHMWPE, led to unidirectional plies (also referred to as “UDs”), cross-plies (also referred to as “X-plies”), and tape fabrics of exceptional anti-ballistic performance, inter alfa arising from the low matrix (glue) content (<8% w/w) required to consolidate the stack of layers. However, the geometrically induced stiffness of UHMWPE tapes, especially on UD, cross-ply and fabric level, entails uncontrollable wrinkling of plies and tapes once draped in or around double curved objects. During moulding, the reinforcing elements, which are generally of larger dimensions than fibres, are constrained on large length scales. As a consequence, irregularities, which may also arise in draw forming, persist upon moulding and lead eventually to lower, uncontrollably inhomogeneous anti-ballistic performance. Moreover, the molecular architecture of most tapes hampers draw forming at temperatures well below the melting temperature.
EP 585 793 relates to a penetration resistant article, e.g. a helmet, comprising a plurality of prepreg packets each comprising at least two prepreg layers wherein said layers are comprised of a fibrous network in a polymeric matrix wherein said prepreg layers have been precompressed into prepreg packets at a temperature and pressure sufficient to bond adjacent surfaces of adjacent layers.
WO 03/074962 relates to a method of making a helmet comprising the steps of cutting a plurality of substantially rectangular, preferably square, blanks from a sheet of resin-impregnated fabric, making curved cuts (denoted by numeral 1 in the Figures of WO 03/074962) in each blank to form a crown portion (5) and lobe portions (3) therefrom, arranging a stack of said sheets into a helmet preform such that the lobe portions of any blank partially overlap adjacent lobe portions of the same blank, and molding the helmet from the preform.
U.S. Pat. No. 3,582,990 relates to a ballistic cover for a protective helmet in which an envelope of relatively light fabric cut and sewed to the shape of the helmet receives an assembly of a plurality of laminates of woven ballistic fabric individually cut and sewed to the shape of the helmet and tacked together around their peripheries with their seams out of line to form the assembly.
WO 2009/047795 relates to a bolt-free helmet comprising a plurality of helmet pre-forms. At least one outer pre-form of the plurality of pre-forms comprises a plurality of slots.
US 2011/0023202 relates to a method of manufacturing a composite laminate comprising the steps of cutting a plurality of ply shapes from prepreg sheet stock and stacking the prepreg ply shapes to form a subassembly of from 2 to 8 cut plies. The subassembly further comprising at least 2 different ply shapes.
GE 2 196 833 relates to a method of making a ballistic helmet in which each of the plies making up the body is formed from a hexagonal blank cut from a ballistic cloth and provided with slits extending from the apices thereof toward the centre to form a central area and segments extending from the central area.
U.S. Pat. No. 5,112,667 relates to an impact resistant helmet, comprising an impact resistant composite shell. The composite shell comprises a plurality of prepreg packets. Each prepreg packet comprises at least about 2 and preferably 5 to 20 prepreg layers. There are from 2 to 50 and preferably 5 to 20 prepreg packets. Each prepreg layer comprises a plurality of unidirectional coplanar fibers embedded in a polymeric matrix. The fibers of adjacent layers in the prepreg packets are at an angle of from 45° to 90°, most preferably about 90° from each other. The prepreg packets are initially flat and are cut into patterns to enable the prepreg packet to be formed into the shape of the shell. The pattern is cut so that upon being formed into the shape of the shell the prepreg packets have substantially no wrinkles. The prepreg packets have cuts or edges which are built in to the shell. The edges substantially come together to form a seam when the packet is formed into the shape of the three-dimensional shell. Adjacent packets formed into the shell have meridial cuts made at different locations on the pattern to avoid overlapping of the seams of adjacent patterns.